Chain link heat treating apparatus and method

ABSTRACT

A system for heat treating chain links including an apparatus and method adapted to heat links one at a time at a desired temperature and for a given duration. The system includes the provision of a pair of electrodes which include clamping means whereby a link can be held by the electrodes during the passage of current through the link. When the desired temperature has been held for a given length of time, the link is released by retraction of the electrodes. A quench tank is preferably located beneath the electrodes whereby a link will drop into the quench tank upon release. Temperature detecting means are preferably employed along with a timing mechanism so that the electrodes will automatically release a link for quenching when the temperature and time factors have been accomplished.

United States Patent [191 Phares et al.

[451 Sept. 30, 1975 CHAIN LINK HEAT TREATING APPARATUS AND METHOD .[73]Assignee: S. G. Taylor Chain Company, Inc.,

Hammond, Ind.

[22] Filed: Jan. 31, 1974 [21] Appl. No.: 438,256

Primary Examiner-J. V. Truhe Assistant ExaminerN. D. Herkamp Attorney,Agent, or Firm-McDougall, Hersh & Scott 5 7 ABSTRACT A system for heattreating chain links including an apparatus and method adapted to heatlinks one at a time at a desired temperature and for a given duration.The system includes the provision of a pair of electrodes which includeclamping means whereby a link can be held by the electrodes during thepassage of current through the link. When the desired temperature hasbeen held for a given length of time, the link is released by retractionof the electrodes. A quench tank is preferably located beneath theelectrodes whereby a link will drop into the quench tank upon release.Temperature detecting means are preferably employed along with a timingmechanism so that the electrodes will automatically release a link forquenching when the temperature and time factors have been accomplished.

17 Claims, 8 Drawing Figures US. Patent Sept. 30,1975 Sheet 1 of 33,909,575

FIG. 2

FIG. 3

US. Patent Sept. 30,1975 Sheet 2 of3 3,909,575

U.S. Patent Sept. 30,1975 Sheet 3 of3 3,909,575

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285m Imam wfi mm? 9% N r wNT/I fizmfimz QQUEZQQ mzcfiz 28:22 3 2i :3 m aNE m3 \finanm Siam $38 5 58 025m: 6328 CHAIN LINK HEAT TREATINGAPPARATUS AND METHOD SPECIFICATION This invention is directed to asystem for heat treating chain links and, in particular, to an apparatusand method for handling individual chain links which are located in alength of chain. The invention finds particular utility in connectionwith the repair of chains used for industrial purposes.

Chain is utilized in industry for numerous purposes. For example,so-called sling chains are widely employed in industry for transmittinggoods throughout plants. Such chains are subjected to heavy loads, andit is, therefore, necessary to maintain the chains in proper conditionin order to avoid down-time of the equipment. More importantly, andparticularly in the case of over-head cranes, the failure of a chainduring operation can lead to injury to workmen and to damage of thegoods being carried as well as to structures over which the goods arebeing transported.

In order to reduce the liklihood of chain failure, legislation has beenenacted requiring periodic inspection of chain. The inspection must bethorough enough to insure that a chain is in proper operating condition,and this requires relatively careful inspection procedures. Inparticular, proper inspection requires removal of the chain from thecrane or other equipment in order that the chain sections can bethoroughly studied and to permit the replacement of any links which donot meet required standards,

The necessity for removing chain from equipment creates significantproblems for users since a new chain must be made available so that theequipment will not have a lengthy down-time. In view of the fact thatregulations require frequent chain inspection, it is necessary toprovide substitute chains for all equipment involved, andthis can leadto significant expense and inventory problems for larger industries.

When inspection procedure reveals that a particular link in a chain isdefective, cutting means are utilized for removing the link. A new linkis then inserted, and secured in the chain length by splice weldingtechniques. In order to insure that the new link will perform properly,heat treating of the new link must follow so that the mechanicalproperties of the link will be acceptable.

Application Ser. No. 290,872, filed on Sept. 21, 1972 and entitled ChainInspection and Repair System, now US. Pat. No. 3,788,062, describes anarrangement for handling the above described procedures in an extremelyefficient fashion.

This invention has a particular object the provision of heat treatingapparatus and method which is highly suitable for use in systems of thetype described in the foregoing application and in operationsindependent of These and other objects of this invention will appearhereinafter and for purposes of illustration, but not of limitation, aspecific embodiment of the invention is shown in the accompanyingdrawings wherein:

FIG. 1 is a front elevational view of a heat treating apparatuscharacterized by the features of this invention;

FIG. 2 is a side elevation of the heat treating apparatus;

FIG. 3 is a fragmentary detail view, partly in section, illustrating theclamping electrodes and drive mechanisms utilized in the construction;

FIG. 4 is a fragmentary detail view illustrating the mechanisms of FIG.3 in elevation;

FIG. 5 is a fragmentary view illustrating the manner in which a singlelink is held by the clamping electrodes;

FIG. 6 is an elevational view of the arrangement shown in FIG. 5;

FIG. 7 is a fragmentary cross-sectional view taken about the line 77 ofFIG. 4; and,

FIG. 8 is a diagrammatic illustration of the control mechanisms utilizedin accordance with the preferred form of the invention.

This invention generally relates to an apparatus and method for heattreating chain links. The apparatus includes spaced apart electrodeswhich include means for clamping a single link between the electrodeswhereby electrical power can be utilized for passing a current throughthe link to achieve a desired temperature in the link for a desiredperiod of time.

The preferred forms of the invention are automated including the meansemployed for releasing a chain link after the desired heating has beenaccomplished. The releasingportion of the cycle is preferably controlledby means for detecting the temperature reached in the link being treatedand by timing means which control the duration of the heating. A quenchtank is located beneath the clamping electrodes so that upon release bythe electrodes, the link will automatically drop into the quench tank.The cycle of operation can conveniently include heat treating at anelevated temperature, for example a normalizing or hardening operation.After quenching, the single link can be reclamped to achieve a desiredtemper and then dropped again into the quench tank. The latter operationis, of course, desirable in order to facilitate handling of the chain.

The accompanying drawings illustrate an apparatus including a mainhousing 10. This housing is provided mainly for a power supply, and atransformer would typically be included in the housing along with othercircuit elements. A control panel is conveniently located on the housingfor purposes of initiating the various operations to be described.

The front panel of the housing defines an opening 12 through whichelectrodes 14 and 16 extend. The ends 18 and 20 of these electrodes areprovided with means for attaching the ends of flexible conductive straps22 and 24. An insulating member 25 is attached to the end 20 ofelectrode 16 so that the strap 22 will at all times be insulated fromthe strap 24.

The other ends of the straps 22 and 24 are connected to supply terminals26 and 28, respectively, with insulation 27 separating strap 22 fromterminal 28. Since, as will be described in greater detail, theelectrodes 14 and 16 are movable, the flexible straps enable continuedconnection of the electrodes to the power supply.

The electrodes 14 and 16 each carry a clamp member 30. The clamp membersinclude V-shaped ends 32 which, as best shown in FIGS. 3 and 5, areadapted to engage a chain link. The clamp members are secured to theelectrodes, preferably by means of threaded fasteners 34 so that theclamp members can be removed. This enables selection of a clamp memberis accordance with the size of a link.

The electrodes 14 and 16 are secured to movable supporting structures bymeans of bolts 36 and 38, respectively. The bolts 36 are received in ablock 40 which is supported on plate 42. Also secured to plate 42 is ablock 44 which serves to secure the end of piston 46 to the plate. Asbest shown in FIG. 4, the end of the piston is received in a member 48and is secured to this member by means of pin 50. The member 48 isreceived by the block 44 and held securely therein.

The bolt 38 extends through the electrode 16 and is received by a block52 which is carried by plate 54. This plate also supports a block 56which receives the end of piston 58. The block 56 is secured to theplate by means of bolts 60, and similar bolts (not shown) are utilizedfor securing the block 44 on the other side of the construction.

The plates 42 and 54 are insulated from the respective electrodes bymeans of insulating sheets 62 and by means of insulating pieces 64. Thebolts 36 and 38 as well as the bolts 66 utilized for holding insulation64 in place are formed with non-conductive materials so that currentpassed to the electrodes will be exclusively directed to the clampingmembers and to a link held between the clamping members.

Cylinders 68 and 70 are associated respectively with the pistons 46 and58. These cylinders are mounted by means of bolts 72 to a support plate74. The plate 74 has thin plates 76 attached thereto as by bolting, andthese plates are in turn secured to a support member 78. The plates 76hold wiper blades 77 of plastic (such as Teflon) which engage slides 92movable therein. The support member 78 is secured to the front panel ofthe housing by means of bolts 80. This front panel 82, as illustratedbest in FIGS. 3 and 7, comprises a relatively heavy plate secured tohousing wall 84 by means of fasteners 86. The fasteners 86 are receivedby spacing members 88 which are welded to the wall portion.

The assembly on the block 78 provides a guide opening constitutedprincipally of openings 90. A slide 92 which is secured to plate 54 bymeans of fasteners 94 is received in the openings 90. A similar slide 96is attached to the plate 42 and a corresponding arrangement of guideopenings is located on this side of the construction. Upon operation ofthe pistons 46 and 58, the slides move in a confined path therebyrigidly controlling the electrode movement.

A mounting member 98 is attached on the opposite side of panel 82 bymeans of bolts 99, and a guide rod 100 is press fit into a bore definedby the member 98. A sliding block 101 defines a central bore whichreceives the rod 100. A pair of pins 102 are secured to the slidingblock 101, and these pins serve as pivotal connections for linking arms104 and 106. The opposite ends of the linking arms are pivotallyattached to pins 108 which extend through the opening 12 in the panel 82for attachment to plates 42 and 54. a corresponding arrangement (notshown) is preferably provided on the other side of the slides (top ofFIG. 4) with linking arms corresponding with the arms 104 and 106 beingpivotally attached on the upper sides of plates 42 and 54 and with aguide rod corresponding with rod 100 extending upwardly along the backof panel 82.

The mechanical structure illustrated results in a controlled movement ofthe clamping electrodes 30. Specifically, the operation of cylinders 68and will result in movement of pistons 46 and 58 whereby the supportplates 42 and 54 will transmit movement to the electrodes. Since thelinking arms 104 and 106 are also connected to the plates 42 and 54,movement of the linking arms and slidingblock 101 must take placesimultaneously with the electrode movement. Because of the location ofthe block 101 along a center line extending between the electrodes, thearrangement insures that the same movement will occur in each electrodeand, therefore, when a link is clamped between the electrodes, the linkmust be centered in the construction.

In the preferred form of the invention, a temperature detecting means112 is located on the panel 82'. This detecting means may comprise,forexample, a Honeywell infra-red detector which is directed along a centerline whereby a link located at the precise center of the constructioncan be observed by the detector.

Any suitable control system may be utilized in conjunction with thetemperature detecting mechanism. This system can be readily set so thata particular heat treating temperature will be realized and thetemperature held for a desired period of time. When this has beenaccomplished, the cylinders 68 and 70 can be automatically operated sothat a link will be released from between the clamping electrodes at aprecisely controlled point in the cycle of operations.

A quench tank 1 14 holding quenching liquid 1 16, for example water, ismounted on the front of the housing. When the clamping electrodesrelease a link, the link will be automatically dropped into the quenchtank without any necessity for handling by an operator of the apparatus.

FIG. 8 provides a diagrammatic illustration of a suitable control systemutilizing a sensor head 112 which may, for example, comprise any deviceadapted to transform infra-red energy detected in the link being treatedto an electrical signal. Such devices include a lens system focused onthe heated link for detecting the infra-red energy and proper alignmentof the link relative to the sensor head is, therefore, an importantaspect of the invention.

The sensor head is connected to signal conditioning unit 122 whichreceives the electrical signal from the sensor head. This unit mayamplify and modulate the signal, eliminating extraneous electricalnoise. Two control features are preferably provided in this unit withone control being set at the upper temperature limit for the hardencycle and the other set at the lower temperature limit for the drawcycle. Accordingly, when the signal'from the sensor, head indicates thata temperature limit has been reached, the unit 122 will develop anappropriate signal.

The unit 122 is connected to relay panel 124 which When an operation isinitiated by the pushbutton panel and when the unit 122 indicates thatthe desired temperature has been reached, the relays control the timeduring which the heat is applied in the respectiive cy-' cles. lt willbe appreciated that the panel 126 will control the operation of onerelay for the-harden cycle and a separate relay for the draw cycle.

After the selected time has expired, the relays are adapted to operatethe ignitron contactor 128 which then shuts off the heating transformer130. The valve 132 is also operated to release the link clamping meanswhereby the heated link will be dropped into the quench tank. 1

The pushbutton panel 126 preferably also has indicator lights toindicate to the operator the particular stage of the cycle. It will beappreciated that variations in the particular components used in thecontrol system are available, and that various conventional means may beemployed for the recited purposes.

In a typical utilization of the described construction, a chain 118 isbrought into position as shown in FIGS. 1, 5 and 6. A link 120 in thechain, usually a repair link which has just been welded, is held betweenthe clamping electrodes, for example with a pair of tongs. It is onlynecessary to locate the link in approximate position, and a pushbuttonor other control member is then utilized for operating the cyclinders 68and 70. The

gization of the power supply whereby the link will be heated throughresistance heating with the detecting means and suitable timingmechanisms determining the magnitude and duration of the heatingoperation; In the preferred operation of the invention, the current iscontinuously interupted at timed intervalsfThe use of a pulsatingcurrent has been found to provide a more uniform distribution of heat tothereby provide more uniform properties in the resulting heat treatedlink. The pistons are then automatically retracted whereby the heattreated link will automatically drop into the quench tank.

In manyinstances, it is desirable to reset a link to achieve a draw ortemper, and the above cycle of operations can simply be repeated butwith theheating operation taking place at a lower temperature. Again, acontrol system can be employed whereby an initial pushbutton operationor the like will automatically. achieve clamping, heating, unclampingand quenching. As noted, the operating cycles are useful with respect tothe manufacture of new chain as well as for repair links. 7

It will be understood that various changes and modifications may bemadein the above described construction which provide thecharacteristics of the invention without departing from thespiritthereof, particularly as defined in the following claims.

That which is claimed is: Y

1. An apparatus for heat treating a single link which is connected in achain made up of a plurality of chain links, comprising a pair of spacedapart electrodes,.said electrodes defining clamp means for clamping asingle chain link in position to span the gap between the electtodeswith the body of the link being substantially centered relative to theelectrodes and being confined thereby, means for supplying electricalpower to the electrodes whereby current is passed through said body ofthe link for heating of the link, separate supports for the electrodes,means for driving at least one of the electrode supports relative to theother between a first position for clamping a chain link and a secondposition for releasing the chain link, including means for automaticallydetecting the temperature reached in a link during heat treating, saiddetecting means being positioned to observe a link held at approximatelythe center line between said electrodes when said supports are in saidfirst position, said means for driving the supports including positioncontrol means whereby said center line is at the same location each timethe supports are moved to the first position, means for controlling themagnitude and duration of the supplying of the electrical power tothereby determine the temperature and time of heat treatment, meansoperatively connected to said detecting means for releasing the linkfrom between the electrodes when the heat treating time has expired, andquenching means for the link, said quenching meansbeing located forreceiving the link after release by the electrodes.

2. An apparatus in accordance with claim 1 wherein said quenching meanscomprises a quench tank positioned beneath said electrodes, said drivingmeans moving the electrodes horizontally between said first and secondpositions, said link dropping vertically into the quench tank uponrelease by the electrodes.

3. An apparatus in accordance with claim 2 wherein the chain portions onopposite sides of the clamped link are suspended over the quench tank.

4. An apparatus in accordance with claim 1 wherein said clamp meanscomprises V-shaped clamp members attached to each electrode for engagingthe opposite side's of a link, and including flexible conductive strapsextending between each electrode and the electrical power supply wherebysaid electrodes can be moved and positioned for the clamping of chainlinks of different sizes while maintaining the connection between theelectrodes and the power supply.

5. An apparatus in accordance with claim 4 wherein said clamp membersare releasibly connected by fastening means to the electrodes to permitchanging of the clamp members for accommodating links of differentdevices.

6. An apparatus in accordance with claim 1 wherein said means fordriving comprises a piston connected to each support, and means forsimultaneously extending and retracting the pistons during movementbetween the firstxand second positions.

7. An apparatus in accordance with claim 1 wherein the means forreleasing the link comprises means connected to said temperaturedetecting means for automatically. unclamping a link when the desiredtemperature is reached and after the link is held at temperature for adesired period of time.

8. An apparatus for heat treating chain links and the like comprising apair of spaced apart electrodes, said electrodes including means forclamping a single chain link in position to span the gap between theelectrodes, means for supplying electrical power to the electrodes,means for driving the electrodes between a first position for clamping achain link and a second position for releasing the chain link, saiddriving means including a piston connected to each electrode, and meansfor simultaneously extending and retracting the pistons during themovement between the first and second positions, at least four linkingarms, each having one end connected to a respective electrode, areciprocating element, the other ends of said linking arms beingconnected to said reciprocating element, the combination of thereciprocating element and the linking arms tying the movement of theelectrodes together, means for controlling the magnitude and duration ofthe supplying of the electrical power to thereby determine thetemperature and time of heat treatment, means for operating said drivingmeans for releasing the link from between the electrodes when the heattreating time has expired, and quenching means for the link, saidquenching means being located for receiving the link after release bythe electrodes.

9. An apparatus in accordance with claim 8 including a guide memberextending vertically beneath and above said electrodes, saidreciprocating member defining a bore receiving said guide member wherebythe reciprocating element is moved up and down as the electrodes aremoved to and from said second position.

10. An apparatus in accordance with claim 9 including movable supportsfor said electrodes, stationary guide means positioned adjacent saidmovable support, and means carried by said movable supports received bysaid stationary guide means for controlling the position of saidelectrodes during movement thereof.

11. An apparatus in accordance with claim 10 including a housing 'forsaid electrical supply, said stationary guide means being mounted on thefront of said housing, and an opening defined by the housing receivingsaid electrode supports, and including flexible conductive stripsattached at one end to said supports and at the other end to said powersupply.

12. A method for heat treating a chain link comprising the steps ofproviding a pair of spaced apart electrodes, inserting a single chainlink between the electrodes, driving the electrodes toward the chainlink to clamp the chain link therebetween, running a current through thechain link to heat the chain link to a predetermined temperature and fora predetermined time, providing means for automatically detecting thetemperature reached in a chain link clamped between the electrodes,detecting the temperature by operating said last mentioned means,providing means for controlling the time at temperature, determining theelapse of a desired time by means of said control means, automaticallyretracting the electrodes when the desired time at temperature has beenreached, and quenching the chain link by locating a quench tank beneaththe electrodes whereby the chain link drops into the quench tank whenthe electrodes are retracted, reinserting said link between saidelectrodes and clamping the link between the electrodes, reapplying acurrent to reheat the chain link to a lower predetermined temperaturefor a predetermined time, said first mentioned temperature comprising ahardening temperature for the link material and said lower temperaturecomprising a drawing temperature for the material, and then againreleasing said link from said electrodes.

l3. Amethod in accordance with claim l2 wherein a length of chain islocated beneath the electrodes with one link thereof being clampedbetween the electrodes, at least portions of the chain length beingdropped into the quench tank along with the chain length being heattreated.

14. A method in accordance with claim 12 wherein said link is heated bya pulsating current passed through the link.

15. A method in accordance with claim 12 wherein a plurality ofindividual links are heat treated and.are thereafter joined together toform a new sling chain.

16. A method for heat treating a chain link comprising the steps ofproviding a pair of spaced apart electrodes, inserting a single chainlink between the electrodes, driving the electrodes toward the chainlink to clamp the chain link therebetween, running a current through thechain link to heat the chainv link to a predetermined temperature andfor a predetermined time, providing means for automatically detectingthe temperature reached in a chain link clamped between the electrodes,providing means for controlling the-time at temperature, automaticallyretracting the electrodes when the desired time at temperature has beenreached, and quenching the chain link by locating a quench tank beneaththe electrodes whereby the chain link drops into the quench tank whenthe electrodes are retracted.

17. A method for heat treating a single linkwhichis connected in a chainmade up of a plurality of chain links, comprising the steps of providinga pair of spaced apart electrodes, said electrodes defining clamp meansfor holding a single chain link in the gap betweenthe.

electrodes, the clamp means for the link including electrodes driveablebetween a first position for clamping the link and a second position forreleasing the link, inserting a chain link between the electrodes,simultaneously driving the electrodes toward the chain link to clamp thechain link therebetween; the body of the link being therebysubstantially centered relative to the electrodes and being confinedthereby, controllingthe,

movement of the electrodes whereby the center of each link treated isalways on a line between and equidistant from each electrode when insaid first position, running a current through the link ,to heat thelink to a predetermined temperature and for a predetermined time, pro

1. An apparatus for heat treating a single link which is connected in achain made up of a plurality of chain links, comprising a pair of spacedapart electrodes, said electrodes defining clamp means for clamping asingle chain link in position to span the gap between the electtodeswith the body of the link being substantially centered relative to theelectrodes and being confined thereby, means for supplying electricalpower to the electrodes whereby current is passed through said body ofthe link for heating of the link, separate supports for the electrodes,means for driving at least one of the electrode supports relative to theother between a first position for clamping a chain link and a secondposition for releasing the chain link, including means for automaticallydetecting the temperature reached in a link during heat treating, saiddetecting means being positioned to observe a link held at approximatelythe center line between said electrodes when said supports are in saidfirst position, said means for driving the supports including positioncontrol means whereby said center line is at the same location each timethe supports are moved to the first position, means for controlling themagnitude and duration of the supplying of the electrical power tothereby determine the temperature and time of heat treatment, meansoperatively connected to said detecting means for releasing the linkfrom between the electrodes when the heat treating time has expired, andquenching means for the link, said quenching means being located forreceiving the link after release by the electrodes.
 2. An apparatus inaccordance with claim 1 wherein said quenching means comprises a quenchtank positioned beneath said electrodes, said driving means moving theelectrodes horizontally between said first and second positions, saidlink dropping vertically into the quench tank upon release by theelectrodes.
 3. An apparatus in accordance with claim 2 wherein the chainportions on opposite sides of the clamped link are suspeNded over thequench tank.
 4. An apparatus in accordance with claim 1 wherein saidclamp means comprises V-shaped clamp members attached to each electrodefor engaging the opposite sides of a link, and including flexibleconductive straps extending between each electrode and the electricalpower supply whereby said electrodes can be moved and positioned for theclamping of chain links of different sizes while maintaining theconnection between the electrodes and the power supply.
 5. An apparatusin accordance with claim 4 wherein said clamp members are releasiblyconnected by fastening means to the electrodes to permit changing of theclamp members for accommodating links of different devices.
 6. Anapparatus in accordance with claim 1 wherein said means for drivingcomprises a piston connected to each support, and means forsimultaneously extending and retracting the pistons during movementbetween the first and second positions.
 7. An apparatus in accordancewith claim 1 wherein the means for releasing the link comprises meansconnected to said temperature detecting means for automaticallyunclamping a link when the desired temperature is reached and after thelink is held at temperature for a desired period of time.
 8. Anapparatus for heat treating chain links and the like comprising a pairof spaced apart electrodes, said electrodes including means for clampinga single chain link in position to span the gap between the electrodes,means for supplying electrical power to the electrodes, means fordriving the electrodes between a first position for clamping a chainlink and a second position for releasing the chain link, said drivingmeans including a piston connected to each electrode, and means forsimultaneously extending and retracting the pistons during the movementbetween the first and second positions, at least four linking arms, eachhaving one end connected to a respective electrode, a reciprocatingelement, the other ends of said linking arms being connected to saidreciprocating element, the combination of the reciprocating element andthe linking arms tying the movement of the electrodes together, meansfor controlling the magnitude and duration of the supplying of theelectrical power to thereby determine the temperature and time of heattreatment, means for operating said driving means for releasing the linkfrom between the electrodes when the heat treating time has expired, andquenching means for the link, said quenching means being located forreceiving the link after release by the electrodes.
 9. An apparatus inaccordance with claim 8 including a guide member extending verticallybeneath and above said electrodes, said reciprocating member defining abore receiving said guide member whereby the reciprocating element ismoved up and down as the electrodes are moved to and from said secondposition.
 10. An apparatus in accordance with claim 9 including movablesupports for said electrodes, stationary guide means positioned adjacentsaid movable support, and means carried by said movable supportsreceived by said stationary guide means for controlling the position ofsaid electrodes during movement thereof.
 11. An apparatus in accordancewith claim 10 including a housing for said electrical supply, saidstationary guide means being mounted on the front of said housing, andan opening defined by the housing receiving said electrode supports, andincluding flexible conductive strips attached at one end to saidsupports and at the other end to said power supply.
 12. A method forheat treating a chain link comprising the steps of providing a pair ofspaced apart electrodes, inserting a single chain link between theelectrodes, driving the electrodes toward the chain link to clamp thechain link therebetween, running a current through the chain link toheat the chain link to a predetermined temperature and for apredetermined time, providing means for automatically detecting thetemperature reached in a chain link clamped between the electrodes,detecting the temperature by operating said last mentioned means,providing means for controlling the time at temperature, determining theelapse of a desired time by means of said control means, automaticallyretracting the electrodes when the desired time at temperature has beenreached, and quenching the chain link by locating a quench tank beneaththe electrodes whereby the chain link drops into the quench tank whenthe electrodes are retracted, reinserting said link between saidelectrodes and clamping the link between the electrodes, reapplying acurrent to reheat the chain link to a lower predetermined temperaturefor a predetermined time, said first mentioned temperature comprising ahardening temperature for the link material and said lower temperaturecomprising a drawing temperature for the material, and then againreleasing said link from said electrodes.
 13. A method in accordancewith claim 12 wherein a length of chain is located beneath theelectrodes with one link thereof being clamped between the electrodes,at least portions of the chain length being dropped into the quench tankalong with the chain length being heat treated.
 14. A method inaccordance with claim 12 wherein said link is heated by a pulsatingcurrent passed through the link.
 15. A method in accordance with claim12 wherein a plurality of individual links are heat treated and arethereafter joined together to form a new sling chain.
 16. A method forheat treating a chain link comprising the steps of providing a pair ofspaced apart electrodes, inserting a single chain link between theelectrodes, driving the electrodes toward the chain link to clamp thechain link therebetween, running a current through the chain link toheat the chain link to a predetermined temperature and for apredetermined time, providing means for automatically detecting thetemperature reached in a chain link clamped between the electrodes,providing means for controlling the time at temperature, automaticallyretracting the electrodes when the desired time at temperature has beenreached, and quenching the chain link by locating a quench tank beneaththe electrodes whereby the chain link drops into the quench tank whenthe electrodes are retracted.
 17. A method for heat treating a singlelink which is connected in a chain made up of a plurality of chainlinks, comprising the steps of providing a pair of spaced apartelectrodes, said electrodes defining clamp means for holding a singlechain link in the gap between the electrodes, the clamp means for thelink including electrodes driveable between a first position forclamping the link and a second position for releasing the link,inserting a chain link between the electrodes, simultaneously drivingthe electrodes toward the chain link to clamp the chain linktherebetween, the body of the link being thereby substantially centeredrelative to the electrodes and being confined thereby, controlling themovement of the electrodes whereby the center of each link treated isalways on a line between and equidistant from each electrode when insaid first position, running a current through the link to heat the linkto a predetermined temperature and for a predetermined time, providingmeans for automatically detecting the temperature reached in the linkduring heat treating, positioning said detecting means to observe thelink at said substantially centered position, controlled the time attemperature, retracting said electrodes when the desired time attemperature has been reached, locating a quench tank beneath theelectrodes, and quenching the link by allowing it to drop into thequench tank when the electrodes are retracted and moved to said secondposition.